All modern portable electronic devices will drain their batteries at some point and either require replacing or re-charging. Attempts to utilize ambient energy by way of energy harvesting systems have not been completely successful. The reasons include the fact that the energy is not available all of the time, such as solar or kinetic. Also by the limitation of the minimum ‘Turn On’ voltage of Silicon transistors which are required for the mass production for use within these devices or the many thousands of transistors within the microprocessor integrated circuit which is limited to 0.7 Volts. Germanium transistors are an alternative as they have a ‘Turn On’ voltage of around 0.3 Volts. However, they are not so readily available and do not lend themselves easily to mass produced integrated circuits (IC) unlike Silicon which is widely used in today's world. The term Threshold Voltage is used to define the ‘Turn On’ or ‘Turn Off’ voltage of transistors.
There are several types of Silicon transistors, the Bipolar Junction Transistor (BJT) and the FET (Field Effect Transistor) being the popular ones. JFETs and MOSFETs (Metal Oxide Field Effect Transistor) are part of the FET family. FETs are different from bipolar transistors in that they have a very high input (Gate) impedance and hence are voltage controlled devices. That is they require a certain voltage to operate but negligible current to drive them rather than Bipolar transistors which require a relatively high current to drive them as they are current controlled devices. FETs are therefore ideal for low power or energy harvesting applications due to their high input impedance and negligible input (Gate) drive current.
Most popular electronic systems use Enhancement Mode MOSFETs due these factors for portable battery powered systems to high power switching applications as they can behave like almost an ideal switch and have a low RDS(On) (Drain Source Resistance) of less than 1 Ohm ranging to just a few micro ohms. In particular they are used in integrated circuits which have thousands or millions of these devices for analogue and digital logic circuits such as microprocessors. The latter of which use a combination of N Channel and P Channel Enhancement Mode MOSFETS. The difference between them is that an N Channel device requires a positive voltage to turn them on while P Channel device requires a negative voltage to turn them on. They are referred to CMOS or Complimentary MOS where they are used together to switch from between low and high logic levels in digital logic integrated circuits and microprocessors. However Enhancement Mode MOSFETs, just like bipolar transistors made from silicon, have a minimum ‘Turn On’ voltage of 0.7 Volts.
Recent attempts have been made to harness electrical energy from ambient energy below these voltages by way of the use of a Depletion Mode FET (Field Effect Transistor) in combination with a step up transformer to form an oscillator to boost the input voltage and charge a capacitor. The capacitor is then used as a power source for the energy harvested powered system. Due to the low IDSS (Drain Source Current) and the relatively high RDS (On), (Drain Source Resistance) of the Depletion Mode FET although outputs of several volts can be achieved, the output power is limited due to the limited current.
There are two types of Depletion Mode FETs: i) the JFET (Junction Field Effect Transistor); and II) the Depletion Mode MOSFET (Metal Oxide Semiconductor Field Effect Transistor). Depletion Mode devices are different from the common Enhancement Mode device as they are ‘On’ by default, acting like a resistor which limits current. In the case of an N Channel JFET, the current is increased by applying a positive voltage across the Gate and Source, or ‘Turn Off’ the device by applying a negative voltage across the Gate and Source of the device. The reverse is true for a P Channel device while the device is still ‘On’ by default. P Channel devices have holes as majority carriers compared to N Channel devices which have electrons hence N Channel Devices are twice as efficient. FIG. 1 shows the FET family free. A JFET is the simplest form of Field Effect Transistor and can be used as an electronically-controlled switch or as a voltage-controlled resistance. Here described is the behaviour based on an N Channel JFET as shown in FIG. 2.
When there is zero voltage applied across the Gate (G) and Source (S) of the N Channel JFET, some current flows through the Drain (D) and Source. However, the resistance across the Gate and Source is relatively high and some current flows. This is called the Zero Gate Voltage Drain Current IDSS. As a positive voltage is applied to the Gate, the resistance drops and allows more current to flow called ‘RDS(On)’. If we reverse the voltage such that there is a negative voltage between the Gate and Source, then the N Channel JFET will stop current from flowing between the Drain and Source. The point at which this happens is called the Gate Source Cutoff Voltage ‘VGS(Off)’. All JFETs therefore have some key operating specifications. For example a typical, cheap easily available N Channel JFET such as a J106 has the following specification: IDSS=200 mA (0.2 Amps) at a VDS of 15 Volts (Drain Source Voltage); RDS(On)=6 Ohms; VGS(Off)=−2 to −6 Volts.
It can be seen from the J106 JFET specification that when there is no Gate signal applied, the IDSS is very low, even with a Drain Source Voltage of 15 Volts. A Depletion Mode MOSFET has similar performance characteristics to the JFET.
Recent attempts have been made utilizing a Depletion Mode MOSFETs or JFETs in combination with a step up transformer to harness ambient energy that provides voltages below 0.7 Volts. Due to the low IDSS and the relatively high RDS (On) although outputs of several volts can be achieved, the output power is limited due to the limited current. One example is a prior art system comprising of an energy harvesting integrated circuit (IC) LTC1308 by Linear Technology Corporation of 1630 McCarthy Blvd. Milpitas, Calif. 95035-7417 as shown in FIG. 3. Here, the harvested energy from the ambient source is captured to charge a small capacitor to up to 3.3 Volts. Here, the ambient source is from a Thermo Electric Generator (TEG) in the form of a Peltier module. The key technology is the step up transformer of a ratio of 1:100 in combination with an N Channel Depletion Mode MOSFET within the LTC3108 integrated circuit which acts like an oscillator to charge a small capacitor which is then used as a power source.
The charger circuit is based on a Forward Converter design. Usually for a Forward Converter an external pulse stream is provided at the Gate of the FET. However, due to the low input voltage, as the N Channel Depletion Mode MOSFET is on at zero volts, when a low voltage is applied, the secondary steps it up by 100 to give around 2 Volts increasing the drain current until again there is an oscillation where the FET turns OFF and ON. The capacitor ‘C2’ is part of a resonant tank circuit between the inductance of the transformer secondary windings, and the input Gate Source capacitance of the FET. This is to allow the oscillating frequency to be towards the self-oscillating frequency of the transformer to allow it to perform at its optimum. The 2 volts or so is also then used as the output to charge the storage capacitor ‘Court’ via the de-coupling capacitor ‘C1’ and the rectifier circuits in the LTC3108 IC. The de-coupling capacitor ‘C1’ in combination with the internal diode across the ‘C1’ terminal and ground of the LTC3108 is used to bring the voltage from the secondary windings positive as the transformer output is an alternating sinusoid on a zero volt axis. The internal rectification and decision making circuits of the LTC3108 decide when to charge the capacitor ‘Cout’ and send a signal to an external microprocessor to inform it that the capacitor is charged and hence it can send data. The device is asserted to have an RDS (On) of 0.5 ohms. The application of the device is limited to remote wireless sensor systems as the charged capacitor is then used as a power source to capture data from sensors, process through a microprocessor and send the data.
FIG. 4 shows using the LTC3108 for a heat based energy harvesting system for a remote wireless sensor application. Here a TEG outputs a voltage of less than 0.1 Volts. The charge time of the capacitor terminated at ‘VOUT’ based on the step up transformer ratio is shown in FIG. 5, it can be seen that any data transmission is therefore not instantaneous as the capacitor requires some time to charge first before it can then be used as a power source for the wireless sensor system. The energy harvesting solution provided by Linear Technology Inc. in the LIC3108 is deemed to be the industries best and coined the phrase ‘The Missing Link for Energy Harvesting Applications’. However, the charger circuit arrangement using a Depletion Mode MOSFET or a JFET in combination with a step up transformer is common. For a Depletion Mode FET, the output power is limited. This solution is impressive as the processing is done using a single IC device. However, the IC is expensive, requires a specific type of step up transformer and requires that the system developer use their device which in turn provides a limited output power that is not instantaneous. It therefore would take hours as a larger capacitor would be needed to be charged so as to send larger amounts of data over a longer range for example.
It is possible to build a discrete energy harvesting system using readily available cheap JFETs such as the J106 N Channel JFET and a step up transformer to perform the step up from less than 0.1 Volts to 3.3 Volts. Hence a LIC3108 would not be required. Any further decision making or power processing circuits to power a microprocessor can then be done using other ICs available on the market. However, the energy harvesting circuit power output would again be limited due to the low IDSS and relatively high RDS(On).
One such example is the ECT310 by Enocean GmbH of Kolpingrin 18a, D-82041 Oberhaching, Germany. This is a module containing discrete components and the same 1:100 ratio step up transformer, the LPR6235-752SMLB by Coilcraft Inc. of 1102 Silver Lake Road, Cary Ill. 60013, USA which is used for the LTC3108 example described in FIG. 4. It is used as a thermal energy powered energy harvester which works with a peltier module. The ECT310 can operate from 20 mV relating to a 2 Kelvin temperature difference provided by the Peltier module. The 20 mV operating voltage is also the same as the example described in the LTC3108 based system of FIG. 4. The ECT310 module is designed for use with wireless sensor networks and restricts use to only the Enocean radio protocol. To this end the ECT310 module can only be used with Enocean's own wireless sensor system such as the STM300 or STM 312 radio module. The Enocean radio module would wake up every 2 minutes to transmit a telegram which requires approximately 5 micro watts, (0.000005 watts) once the ECT310 module has charged a small capacitor between 3 to 5 Volts. Hence the power is limited for small bursts of data of only 5 microwatts every two minutes and not instantaneous data output which would require significantly greater power output.
It is also possible to use recently available Zero Threshold MOSFETs instead. They potentially have many advantages in energy harvesting applications due to their zero threshold voltage and hence can start operating without a need for a step up transformer. However, presently, they are depletion mode MOSFETS. Therefore they would serve the same purpose as the JFET or standard Depletion Mode MOSFET in terms of power output. Their output is significantly less than Enhancement Mode MOSFETs due to their higher RDS.
U.S.Pat. No. 2002074898 (A1) discloses a ‘Self-powered wireless switch’ powered by a piezo electric switch driving an RF transmitter. A paper written by Joseph A. Paradiso and Mark Feldmeier entitled ‘A Compact, Wireless, Self-Powered Pushbutton Controller’ shows the same circuit in more detail used for the application for an RF ID Tag. Here a piezo element is struck by way of a piezo electric switch which generates 2000 volts similar to a cigarette lighter. This is then stepped down through a step down transformer and charges a 4.4 μF (micro Farad) capacitor that is regulated to 3 Volts and then encoded by a 12-bit encoder IC, HT12E before being transmitted using an RF (Radio Frequency) transmitter. This is illustrated in FIG. 6. The output is instantaneous, however there are similar limitations for both patent US 2002074898 (A1)) and the paper entitled ‘A Compact, Wireless, Self-Powered Pushbutton Controller’. Firstly it requires the pressing of a piezo electric switch to generate the electrical power which will wear over time. Secondly, the output power is limited in that it can only charge a 4.4 μF (micro Farad) Capacitor to power an encoder and a RF transmitter to 3 Volts, and transmit a 12-bit digital code sequence for a limited 30 ms (Milliseconds). Graphs of the capacitor voltage after striking the piezo electric switch and the transmission are illustrated in FIG. 7.
For the ID tag application it is aimed for, it has limitations due to the limited 12-bit digital code sequence (due to the frequency limitations and the 30 ms transmit time available) which means only 4096 codes can be obtained. These codes can only be programmed by hard wiring each of the encoders in 4096 separate units. The button also needs to be pressed to enable it to operate.